1. Field of the Invention
The present invention relates to a method and a device for controlling power of pulsed light emitted by a light source, condensed by a object lens and irradiated to a recording medium for recording information in the recording medium. More specifically, the present invention relates to determination of a peak power and a bottom power in the information record using pulsed light.
2. Description of the Prior Art
Recently, request for high record density and high speed of data transmission has become stronger and stronger in the information recording device using an optical disk or a magneto-optical disk.
In order to record information in a recording medium such as a magneto-optical disk, it is necessary to condense a laser beam from a light source such as a laser diode using an object lens and irradiate the laser beam to the recording medium. In the case of a magneto-optical disk, a recording magnetic film at a portion irradiated with the laser beam is heated to a temperature above the Curie point, so that the recording magnetic film is magnetized in the vertical direction by the magnetic field applied externally. A bit information "0" or "1" is recorded in accordance with the direction of the magnetizing. In the case of a phase-change type optical disk, a recording film at a portion irradiated with the laser beam is changed from crystalline state to amorphous state by being heated and cooled, so that a pit corresponding to a bit information "0" or "1" is formed.
There are two methods for recording information in the magneto-optical disk. One is light modulation method, and another is magnetic field modulation method. In the light modulation method, the magnetized direction of the recording magnetic film in the recording area is aligned in one direction by erasing process, and then only the portion of the recording magnetic film irradiated with laser beam is magnetized in the opposite direction. Namely, the portion irradiated with laser beam and other portions have different magnetized direction to each other.
The magnetic field modulation method uses a continuous light or a pulsed light to irradiate the recording magnetic film, while alternating the direction of the magnetic field that is applied to the recording magnetic film in accordance with the record information ("0" or "1"). Thus, the magnetized direction of the recording magnetic film is alternated in accordance with the recorded information. The magnetic field modulation method can be adapted to overwriting, so that the erasing step for aligning the magnetized direction of the recording magnetic film in the recording area is not necessary.
In each method for recording information mentioned above, the power of the irradiated light must be controlled and optimized when the laser beam is condensed and irradiated to the recording medium. If the irradiation power, i.e., the light emission power is too low or too high, an adequate C/N ratio (the ratio of carrier and noise) and a predetermined error rate cannot be obtained.
One method that has been used for satisfy the above-mentioned condition is to perform test writing with a predetermined light emission power before writing data, so as to determine the optimum power in accordance with the error rate obtained by the read signal after the test writing as disclosed in Japanese Unexamined Patent Publication No. 6-212610(A), for example. In the case of pulsed light recording, the bottom power is usually preset to a value nearly equal zero, and the peak power is increased step by step from relatively small value, so as to repeat the test writing and the reading.
FIG. 5 shows an example of the bottom power Pb, the peak power Pw, the emission periods Ts and Tp in the case of pulsed light emission. If the bottom power Pb is zero, the light emission duty is given by the expression Tp/(Tp+Ts). Hereinafter, the light emission duty is assumed to be given by the expression Tp/(Tp+Ts) also in the case where the bottom power Pb is not zero.
FIG. 6 is a flow chart showing the conventional method for determining the power of the irradiated light. In this example, on the precondition that the light emission duty is fixed, the bottom power is preset (fixed) to a predetermined value in Step #601, the peak power increases step by step in Step #605, while the test writing in Step #602 and the read check in Step #603 are repeated. For example, starting from a relatively low peak power in Step #601, the peak power is increased step by step until a good result of read check (the optimum condition) after the test writing is obtained in Step #604. Thus, the optimum peak power, the bottom power and the light emission duty (i.e., the optimum light emission power) is determined in Step #606.
If the optimum condition is not obtained after increasing peak power to a possible upper limit, the emission period Ts of the peak power Pw may be increased so as to enlarge the light emission duty. Then, the test writing and the read check may be repeated.
However, when determining the optimum light emission power by the above-mentioned conventional method, the peak power can be insufficient because of the following reasons.
First, the maximum power of the laser diode that can be used for the product is limited. In addition, if the disk recording medium rotates in a constant rotation speed, peripheral speed in the outer portion of the disk recording medium is faster than that in the inner portion, and a larger peak power is required in the outer portion.
Furthermore, since a small condensed light spot is required along with recent high recording density, only center portion of the laser beam emitted by the laser diode is used after condensed by the object lens, while the peripheral portion of the laser beam is not used. In other words, there is a tendency to increase the vignetting of the object lens so as to obtain a smaller light spot, resulting in decrease of efficiency of condensing.
There is also a limit in the method of increasing the emission period of the peak power and enlarging the light emission duty so as to increase the light emission power. Namely, when the emission period of the peak power is increased, heat distribution becomes extended, so a cross write between neighboring tracks may occur, or an edge deterioration of the recorded mark may be easily generated.
As a result, reading characteristics and an error rate may be deteriorated.
Because of the above-mentioned factors, the peak power may become insufficient. Other factors such as a variation of ambient temperature also can cause the insufficient peak power. In order to get sufficient error rate at the upper limit of the peak power, the rotation speed as well as the peripheral speed of the disk recording medium must be lowered. However, the lowered speed may cause a problem that the device is not capable of the high speed data transmission.